1. Field of the Invention
This invention generally relates to an electromagnetic clutch, and more particularly, to a mechanically reinforced rotor which can cope with increased tension in the driving belt.
2. Description of the Prior Art
Electromagnetic clutches are well known in the prior art and are often used for controlling the transfer of power from an automobile engine to the refrigerent compressor of an automobile air conditioning system. One such electromagnetic clutch is disclosed in Japanese Utility Model Application Publication No. 63-42936 which correspondes to U.S. Pat. No. 4,892,176.
The construction of an electromagnetic clutch disclosed in the '936 Japanese Utility Model is shown in FIG. 1. With reference to FIG. 1, the clutch assembly is disposed on the outer peripheral portion of annular tubular extension 11, which projects from an end surface of compressor housing 10 to surround drive shaft 20. Drive shaft 20 is rotatably supported in compressor housing 10 by a bearing (not shown). The clutch assembly includes rotor 30 rotatably mounted on tubular extension 11 by bearing 12.
Rotor 30 is coupled to the automobile engine (not shown) through a belt (not shown). Rotor 30 has a U-shaped configuration in its sectional side view, and includes outer cylindrical portion 31, inner cylindrical portion 32, and axial end plate portion 33. End plate portion 33 connects the outer and inner cylindrical portions 31 and 32 at one axial end. Axial end plate portion 33 is provided with a plurality of concentric arcuate slits 301, 302, which are disposed on outer and inner concentric circles, respectively. Slits 301, 302 are angularly synchronized and oriented of equiangular intervals as shown in FIG. 3. This construction forms magnetic pole face 303.
Hub 40 is fixed to the outer terminal end of drive shaft 20 extending beyond tubular extension 11. Armature plate 50 is movably connected to hub 40 by a plurality of leaf springs 60. As discussed below, leaf springs 60 permit axial movement of armature plate 50 toward pole face 303. Leaf springs 60 are fixed to the outer surface of armature plate 50 by rivets 51. The axial end surface of armature plate 50 faces pole face 303 of rotor 30 with a predetermined axial air gap "G" therebetween. The axial end surface of armature plate 50 is provided with concentric arcuate slits 501, forming pole face 502. Slits 501 are positioned to be opposite the midway point between slits 301, 302 on pole face 303.
Electromagnet 70 is mounted on compressor housing 10 concentric with drive shaft 20. Electromagnet 70, including the electromagnetic coil 71 provided therewithin, is disposed within annular hollow portion 304 of rotor 30 such that is surrounded by an air gap. When coil 71 of electromagnet 70 is energized, pole face 502 is attracted to pole face 303. Thus, drive shaft 20 rotates as rotor 30 is turned by the engine. If coil 71 of electromagnet 70 is not energized, pole face 502 of armature plate 50 is separated from pole face 303 of rotor 30 by the recoil strength of leaf springs 60. Rotor 30 still rotates in reponse to the engine output, but drive shaft 20 is not turned.
In the above construction of the electromagnetic clutch, magnetic flux "M", which is produced around electromagnet 70 by the energizing of electromagnetic coil 71, passes through a magnetic passageway formed within electromagnet 70, rotor 30 and armature plate 50 as indicated by the dotted line FIG. 1.
In recent years, in order to effectively utilize the restricted vacant space in an automobile engine compartment, the respective pulleys or rotors associated with the automobile engine and devices deriving power from the engine (such as, an automobile generator, an oil hydraulic pump used for a power assisted steering wheel, and the refrigerant compressor) receive only one driving belt as shown in FIG. 2. With reference to FIG. 2, pulleys 300, 400, and 500 are associated with the automobile engine, the automobile generator and the oil hydraulic pump, respectively. Rotor 30 is one of the rotatable members of the electromagnetic clutch which is associated with the refrigerant compressor. Pulley 600 is a so-called "idle pulley", which is used for adjusting the tension in driving belt 200. Driving belt 200 is sequentially received by pulley 300, pulley 400, rotor 30, pulley 500, and pulley 600. When driving belt 200 is received by pulleys 300, 400, 500 and 600 and rotor 30, the tension of driving belt 200 is adjusted to an increased level to avoid a reduction in the capacity of transferring the driving power from the automobile engine to the automobile generator, the oil hydraulic pump and the refrigerant compressor.
Specifically, an increased tension in the driving belt generates a force which seriously tends to nutate rotor 30. As a result of the rotor'U-shaped configuration undesirable metal fatigue occurs at a certain circumferential portion "F" of axial end plate portion 33 of rotor 30. The circumferential portion F lies adjacent the inner circular arc of slits 302 disposed on the inner concentric circle. As can be appreciated, such fatigue causes serious damage to rotor 30.
In order to resolve this drawback, rotor 30 must be mechanically reinforced. However, when rotor 30 is mechanically reinforced by increasing the thickness of axial end plate portion 33, the increased weight of rotor 30 will be above a negligible amount. In addition, an increase in the thickness of axial end plate portion 33 makes the blanking process for forming slits 301 and 302 more difficult. Furthermore, when rotor 30 is mechanically reinforced by increasing the arcuate distance of each remaining region 301a, 302a located between adjacent arcuate ends of slits 301, 302, a resulting decrease of magnetic attraction force acting between pole faces 502 and 303 will be above a negligible amount.